Our objective is to develop an energy selective cassette for dual energy X-ray imaging. The cassette will use the technique of active modulation of luminescence gain invented by Aprend Technology. This approach has the promise of providing dramatic improvements in performance while being based on existing computed radiography detector technology. It uses photostimulable luminescent storage phosphor screens that can be read by the laser scanners of computed radiography systems. The data can then be processed by the system's computer. Dual energy X-ray imaging has the potential to increase significantly the diagnostic power of medical X-ray examinations. With it, the chemical composition of materials can be calculated from the X-ray data. Tissues can be selectively removed from the image based on their composition. By removing obscuring background structures, the detectability of medically important features is improved. The technique can enhance the visibility of microcalcifications in mammography or characterize calcium in solitary pulmonary nodules detected in chest radiography. The Phase I research developed two innovative approaches to energy selective cassettes using active modulation of luminescence gain. The specific aim of the Phase II research is to study these approaches, to implement prototypes, and to measure their performance in dual energy imaging.